To power higher and higher performance aircraft, the aircraft gas turbine industry is expending significant effort in attempting to increase the thrust-to-weight ratio of its engines. Greater thrust can be achieved in part by increasing the burner temperature rise, and weight reduction is obtained in part by shortening the combustor. One vehicle for attaining increased temperature rise without sacrificing flame stability is a double annular burner, exemplified in U.S. Pat. No. 4,194,358, granted to R. E. Stenger on Mar. 25, 1980. However, while double annular burners are capable of increased temperature rise, they have the disadvantage of increasing dome height and combustor surface area, which increases engine weight.
In both double annular and conventional single annular burners, a major factor limiting the combustor volumetric heat release rate is the rate of mixing between fuel and air. Fuel-injectors/air-swirlers in the combustor dome are quite effective mixing devices, but regions of relatively low mixing, and therefore low volumetric heat release, exist in areas adjacent to the swirlers. Because the combustor interior is not uniformly occupied by high intensity burning, a longer combustor is needed to ensure combustion is complete before the product gases enter the turbine.
Swirl orientation could be used to improve the uniformity of high intensity combustion in single and double annular burners, but normally it is not so used, for combustor durability reasons. If adjacent swirlers had alternating swirl orientations, e.g., first swirler: clockwise, second swirler: counterclockwise, third swirler: clockwise, etc., the fuel-air mixture near the periphery of one swirler would always be flowing in the same direction as the peripheral flow from the adjacent swirler. Thus, the two swirlers would reinforce one another, increasing the mixing and burning rate between swirlers. However, combustors with all swirlers at a constant radius, or double annular combustors with two rows of swirlers separated by an intervening wall, usually have the same swirl orientation for all swirlers because of the need to avoid thermal distress of the burner liner. With all swirl orientations the same, flow at the periphery of one swirler opposes the direction of flow near the edge of the adjacent swirlers, thereby preventing hot gases from any swirler from impinging on the combustor walls. The deleterious side effect of a common swirl orientation is the creation of low intensity combustion regions between swirlers.